This commit fixes the description of a couple of multirange operators and
oprjoin for another multirange operator.  The change of oprjoin is more
cosmetic since both old and new functions return the same constant.

These cosmetic changes don't worth catalog incompatibility between 14beta2
and 14beta3.  So, catversion isn't bumped.

Discussion: https://postgr.es/m/CAPpHfdv9OZEuZDqOQoUKpXhq%3Dmc-qa4gKCPmcgG5Vvesu7%3Ds1w%40mail.gmail.com
Backpatch-throgh: 14